1. Field of the Invention
This invention is related in general to the fields of stereotactic radiosurgery and radiation therapy. In particular, the invention provides a new method and apparatus for producing a precise set of coordinates of the portion of a patient's body affected by a tumor with reference to a fixed frame wherein the patient is immobilized, so that the required dosage of radiation can be accurately delivered to the prescribed target volume with substantial sparing of surrounding normal tissues.
2. Description of the Related Art
The main object of radiotherapy is to deliver the prescribed dose of radiation to a tumor in a patient while minimizing the damage to surrounding, healthy, tissue. Since very-high-dose radiation (in the order of several thousand rads or cGy, typically generated by a linear accelerator) is normally used to destroy tumors in radiotherapy, the high dose is also destructive to the normal tissue surrounding the tumor. Therefore, it is essential that the delivery of radiation be limited precisely to the prescribed target volume (i.e., the tumor plus adequate margins). This is normally accomplished by placing appropriately constructed shielding blocks in the path of the radiation beam. Thus, the goal is to accurately identify the malignancy within the body of the patient and to target the prescribed dosage of radiation to the desired region in the immobilized patient.
To that end, the ideal procedure requires the identification of the exact anatomical location of the tumor and the corresponding accurate positioning of the radiation field during treatment. This could be easily achieved if it were possible to locate and treat the tumor at the same time. In practice, though, this is not possible because the equipment used to identify the tumor (x-ray machine, computed tomography equipment, or any of the other scanning machines currently in use) is separate from the equipment used for the therapeutic irradiation of the patient, requiring the movement and repositioning of the patient from one piece of equipment to the other.
As illustrated in schematic form in FIG. 1, a conventional treatment unit 10 consists of a linear accelerator (linac) head 2 mounted on a gantry 4 so that its high-energy emissions R irradiate a patient P lying on a table 6 directly below, typically through shielding blocks 8 attached to the head. A bracket 12 supporting a detector 14 may be mounted on the opposite side of the head within the field of radiation in order to take radiographs of the patient being treated. The gantry 4 is movable around a pivot 16 to permit the rotation of the head (and of the detector) around the patient to afford different views of the area to be treated ("multiple fields" treatment).
The normal procedure involves the use of a diagnostic simulator, which is a diagnostic x-ray machine with the same physical characteristics of the radiation therapy machine (schematically also represented by FIG. 1, where a diagnostic x-ray head replaces the linac head 2), so that the field of view of the low-energy x rays emitted in the simulator is the same as that of the high-energy radiation emitted in the radiation therapy machine. Prior to treatment, the patient is radiographed using the simulator and an image of the target area is obtained with low-energy radiation, which yields good image quality. The exact target volume is then delineated on the radiograph by a physician and matching shielding blocks are constructed to limit the field of view of the irradiating machine to the region so delineated.
A different approach has been used in the field of cranial radiosurgery, which requires very precise high-intensity radiation delivered in a single session. Rather than irradiating the target through shielding blocks, which provide only a coarse alignment of the tumor area with the field of emission, cranial radiosurgery relies on a highly focussed stereotactic radiation beam pointed precisely toward the center of the tumor. In order to be able to direct the radiation with sufficient accuracy, a cranial frame consisting of a rigid ring is affixed to the skull of a patient below the tumor area by means of at least four pressure pins evenly distributed around the ring. The pins compress the bone to the point of becoming rigidly affixed to the skull, thus providing a fixed frame of reference for delineating the position of the malignancy. With the use of scanning equipment, such as a CT scanner, the exact location of the tumor can thus be mapped in terms of three-dimensional coordinates in relation to the ring of the cranial frame. Once these exact coordinates are known, the patient is moved to the linac machine where the cranial frame is lined up with a special cranial support calibrated to the machine's own reference system and the frame is positioned so that the stereotactic radiation beam is focussed on the center of the tumor. Thus, as the linac's gantry rotates around head of the patient for multiple-fields treatment, the tumor is subjected to the cumulative amount of radiation emitted during the radiation session, while the areas surrounding the tumor receive only the radiation emitted while the path of the beam passed through them.
This technique requires precise measurements and targeting of the radiation beam, but it is indispensable for the treatment of cranial tumors, where even a slight misalignment of the radiation beam may cause severe damage to surrounding vital tissue. Therefore, it would be very desirable to have an apparatus that permitted the use of a similar technique for extracranial applications. The present application is directed at the development of a device that enables the use of this technique for stereotactic localization and radiation therapy of extracranial targets.